For which particular post you want to apply in Natural Resources Defense Council as you did not mention, here I will gather details.

Have a glance on this process

Look on to the site of Natural Resources Defense Council which looks like this

Natural Resources Defense Council

Look for section of career
Which is at middle bottom page then click on it
A new page will open there click the option of open positions
A new page will open which looks like this
There fill the required information